For some years, tire manufacturers have been making particularly great efforts to develop novel solutions to a problem dating from the very beginning of the use of wheels fitted with tires of inflated type, namely how to allow the vehicle to continue its journey despite a significant or complete loss of pressure from one or more tires. For decades, the spare wheel was regarded as the sole and universal solution. Then, more recently, the considerable advantages related to its possible removal have become apparent. The concept of “extended mobility” was developed. The associated techniques make it possible to run with the same tire, according to certain limits to be observed, after a puncture or a fall in pressure. This makes it possible, for example, to drive to a breakdown point without having to halt, under often hazardous conditions, to fit the spare wheel.
Self-sealing compositions capable of making it possible to achieve such an objective, by definition capable of automatically ensuring, that is to say without any external intervention, the airtightness of a tire in the event of perforation of the latter by a foreign body, such as a nail, are particularly difficult to develop.
In order to be able to be used, a self-sealing layer has to satisfy numerous conditions of a physical and chemical nature. In particular, it has to be effective over a very wide range of operating temperatures, this being the case over the whole of the lifetime of the tires. It has to be capable of sealing the hole when the perforating article remains in place; when the latter is expelled, it has to be able to fill in the hole and to render the tire airtight.
Numerous solutions have admittedly been devised but they have not been able truly to be developed to date in vehicle tires, particularly due to difficulties in manufacturing these self-sealing compositions and thus to their final cost price.
In particular, high-performance self-sealing compositions based on natural rubber and hydrocarbon resin as tackifying agent (tackifier) have been described in Patents U.S. Pat. No. 4,913,209, U.S. Pat. No. 5,085,942 and U.S. Pat. No. 5,295,525. These compositions are characterized by the combined presence of a high content of hydrocarbon resin, always greater than 100 parts by weight per hundred parts of solid elastomer, and of a large amount of elastomer in the liquid state, generally in the form of depolymerized natural rubber (molecular weight typically of between 1000 and 100 000).
First of all, such a high resin content, apart from the fact that it can be harmful to the hysteresis and consequently to the rolling resistance of the tires, requires a particularly lengthy and difficult kneading of the elastomer matrix.
The use of a large amount of liquid elastomer admittedly improves the fluidity of the composition but such a use is the source of other disadvantages, in particular of a risk of creep of the self-sealing composition during use at relatively high temperature (typically greater than 60° C.) frequently encountered during the use of some tires.
Another major manufacturing problem may also arise: in the absence of filler, such as carbon black, or at the very least of a significant amount of such a filler (furthermore undesirable, in a known way, for this type of application), the composition exhibits weak cohesion. This lack of cohesion may be such that the adhesiveness of the composition, resulting from the high content of tackifying resin employed, is no longer compensated for and prevails. This then results in a risk of undesirable adhesive bonding to the compounding equipment, which is unacceptable under industrial processing conditions.